It is known in the art to use a pot-shaped electromagnet for driving the blades of a photographic shutter (Mielke U.S. Pat. No. 3,724,350, granted Apr. 3, 1973) and for adjusting the leaves of an iris diaphragm (Mielke U.S. Pat. No. 3,812,501, granted May 21, 1974). The present invention is in the nature of an improvement in the means for providing electric current to drive the electromagnet, particularly when the electromagnet is used for adjusting a diaphragm rather than for driving shutter blades.
When the diaphragm of the camera is adjusted by means of an electromagnet, a convenient way to make such adjustment automatic in accordance with prevailing light conditions is to have the light fall on a photo-converter or photocell electrically connected to the circuit of the electromagnet. The photo-converter may be placed behind the diaphragm, receiving its light through the diaphragm, so that as the diaphragm closes down, less light is received by the photo-converter, and as the diaphragm opens wider, more light is received. This arrangement is particularly suitable for use in a single lens mirror reflex camera. Examples of arrangements where the light coming through the diaphragm falls upon a photo-converter behind the diaphragm, are disclosed in the present applicant's U.S. Pat. No. 3,792,485, granted Feb. 12, 1974, and the present applicant's copending application, Ser. No. 667,596, filed Mar. 17, 1976 as a continuation of Ser. No. 539,379, filed Jan. 8, 1975, which was a continuation of Ser. No. 410,251, filed Oct. 26, 1973.
In a known form of single lens mirror reflex camera of this general kind, the diaphragm driving ring is connected to the plunger coil of the pot-shaped magnet, approximately in the manner disclosed in the above mentioned U.S. Pat. No. 3,812,501. A photoelectric converter situated behind the diaphragm and receiving light through the diaphragm supplies an exciter current, independent of the exposure, to the plunger coil of the pot-shaped magnet. The coil enters into or emerges from the pot-shaped magnet, according to the direction in which the current flows. The distance to which it enters or emerges depends on the magnitude of the exciter current, so that by regulating the exciter current, the diaphragm can be set to a certain aperture. The exciter current is regulated by means of an exposure regulator which measures the light passing through the diaphragm aperture and falling on the photoelectric converter, and compares this value with a nominal value, and conveys an appropriate exciter current to the plunger coil, through a power amplifier.
In addition to the power loss involved in the power amplifier, the main disadvantage of this method of actuating the diaphragm is that, in the vicinity of the balancing point of the diaphragm, the plunger coil is provided with only a very low exciter current, its controlling force thus being very limited. In many cases this low controlling force is insufficient to overcome the mechanical static friction of the diaphragm adjusting system, so that the diaphragm does not move through slight adjusting movements that should take place, and a slight deviation from the theoretically correct diaphragm aperture will prevail.
A further difficulty with the prior arrangement is the problem of braking the movement of the diaphragm mechanism when the correct diaphragm setting is reached, to prevent the diaphragm from overshooting or of oscillating about its balancing point.
An object of the present invention is to provide a diaphragm drive control system which will insure a very accurate diaphragm setting, even with the use of a very limited controlling force.
Another object is the provision of such a diaphragm drive control system which is particularly satisfactory and trustworthy when making slight adjustments in the vicinity of the balancing point.
Still another object is the provision of a diaphragm drive control system so designed that during the adjusting movement, the moving parts approach the theoretically correct setting asymptotically.
A further object is the provision of a diaphragm drive control system so designed that the heat developed in the amplifier stage of the exciter current is reduced to a minimum.